As electronic devices are getting less expensive, more compact, more sophisticated and more energy-efficient, switching power supply units are strongly expected to be smaller, highly efficient and to have stable output characteristics. In recent years, semiconductor components, particularly microprocessors, have been having lower voltage and larger current. Along with this trend, in order to secure the supply of more stable voltage and larger current, there is now a transition from the concentrated power-supply system to the distributed power-supply system in which power is supplied in the vicinity of semiconductor devices such as semiconductor components. In the distributed power-supply system, it is necessary to convert from a comparatively high bus voltage of, e.g. 48V used for supplying power to each unit of an electronic device to a highly stable low voltage of, e.g. 1V necessary for the operation of the semiconductor devices such as the semiconductor components.
A switching power supply unit allows a switching element repeatedly turning on and off to generate rectangular alternating voltage; a high-frequency transformer or the like to convert the alternating voltage; and a rectifier and a smoothing circuit to further convert the converted alternating voltage into direct current. The transformer used here is composed of a magnetic body around which a primary winding and a secondary winding are wound several times. The voltages to be applied and induced on the windings can be changed by adjusting the number of turns of the windings.
In switching power supply units, it is general that rough conversion of voltage is done by a transformer, and fine adjustments are made by PWM (Pulse Width Modulation) controlling the on-off ratio of the switching element. The primary and secondary windings used in the transformer are chosen mainly by the voltages to be applied thereon, and a higher voltage requires a larger number of turns. However, an increased number of turns of windings in the transformer increases the volume necessary for the insulation between the windings, thereby increasing the overall size of the transformer.
One such conventional switching power supply unit which has achieved reduction in thickness and size is shown in FIGS. 5 to 7. FIG. 5 is a circuit block diagram showing the structure of a conventional switching power supply unit; FIGS. 6A to 6E are signal waveforms of main parts of the conventional switching power supply unit; FIG. 7 is a diagram of a transformer of the conventional switching power supply unit.
In the case where the switching power supply unit uses a multilayer printed board, an increase in the number of turns of windings must be achieved by either increasing the number of turns per layer or the number of layers. The following is a description about a half-bridge converter, which is a typical conventional switching power supply unit. Half-bridge converters are known as a circuit system capable of reducing the voltage to the windings of the transformer.
In FIG. 5, input voltage 1 (Vin) is placed between input terminals 1a and 1b. A capacitor series circuit of first capacitor 2 and second capacitor 3 is connected between input terminals 1a and 1b. A series circuit of first switching element 4 and second switching element 5 is connected with input terminals 1a and 1b. Transformer 6 includes primary winding 6a, first secondary winding 6b and second secondary winding 6c. One end of primary winding 6a is connected to the common connection point of first and second capacitors 2 and 3, and the other end is connected to the common connection point of first and second switching elements 4 and 5. First secondary winding 6b and second secondary winding 6c are connected in series. Second secondary winding 6c of transformer 6 and first rectifier 7 are connected in series. Second secondary winding 6c of transformer 6 and second rectifier 8 are connected in series. The cathodes of first rectifier 7 and second rectifier 8 are commonly connected to each other. First and second rectifiers 7 and 8 full-wave rectify the voltages generated in first and second secondary windings 6b and 6c of transformer 6.
Inductance element 9 and smoothing capacitor 10 are connected in series, and the other end of inductance element 9 is connected to one end of smoothing capacitor 10. The other end of smoothing capacitor 10 is connected to the common connection point of first and second secondary windings 6b and 6c of transformer 6. What is called a smoothing circuit formed of inductance element 9 and smoothing capacitor 10 smoothes full-wave rectified voltages obtained in first and second rectifier 7 and 8, and generates stable voltages at both ends of smoothing capacitor 10.
Output terminals 11a and 11b output voltages generated at both ends of smoothing capacitor 10. Load 12 is supplied with stable output voltages Vout obtained from smoothing capacitor 10. Control circuit 13 detects the voltages outputted from output terminals 11a and 11b, and in order to stabilize the output voltages, determines the on-off ratio (hereinafter, time ratio D) between first switching element 4 and second switching element 5 so as to drive these elements.
Of FIGS. 6A to 6E, FIG. 6A is a driving waveform of first switching element 4; FIG. 6B is a driving waveform of second switching element 5; FIG. 6C is a waveform of voltage developed in primary winding 6a of the transformer; FIG. 6D is a waveform of full-wave rectified voltage applied to the series circuit of inductance element 9 and smoothing capacitor 10; and FIG. 6E is a waveform of current flowing through inductance 9.
First and second switching elements 4 and 5 are turned on and off alternately at an equal time ratio of D<0.5. Turning first and second switching elements 4 and 5 on and off in an equal time ratio of D allows first and second capacitors 2 and 3 to divide the input voltage into half, that is, Vin/2 each.
Turning on first switching element 4 applies primary winding 6a of transformer 6 with voltage Vin/2 of first capacitor 2. Letting the turns ratio between the primary winding and the secondary windings be N, first and second secondary windings 6b and 6c of transformer 6 each develop voltage Vin/(2N). This causes first rectifier 7 to turn on and second rectifier 8 to turn off, thereby applying the series circuit of inductance element 9 and smoothing capacitor 10 with voltage Vin/(2N).
Similarly, turning on second switching element 5 applies primary winding 6a of the transformer with voltage Vin/2 in the opposite direction, and further applies first and second secondary windings 6b and 6c of transformer 6 with voltage Vin/(2N) in the opposite direction. This causes first rectifier 7 to turn off and second rectifier 8 to turn on, thereby applying the series circuit of inductance element 9 and smoothing capacitor 10 with voltage Vin/(2N).
When first and second switching elements 4 and 5 are both off, transformer 6 has a winding voltage of zero, thereby turning on first and second rectifiers 7 and 8. At this time, the series circuit of inductance element 9 and smoothing capacitor 10 has a voltage of zero. The current of inductance element 9 is divided into first rectifier 7 and second rectifier 8. The output voltages are determined by time ratio D of first and second switching elements 4 and 5, and the turns ratio (primary winding: secondary winding=N:1) between primary winding 6a and the secondary windings of transformer 6 so as to make output voltages Vout=D(1/BN)Vin.
In the half-bridge converter, an input voltage is divided by first and second capacitors 2 and 3 to reduce the voltage to be applied to primary winding 6a of transformer 6, so that turns ratio N between the primary and secondary windings can be comparatively small. In order to obtain predetermined output voltages Vout, turns ratio N and time ratio D can be arbitrarily determined. However, a reduction in turns ratio N leads to an increase in the voltage to be developed in secondary windings 6b and 6c of transformer 6. As a result, a larger voltage is applied on first and second rectifiers 7 and 8 to make it necessary to prepare a device highly resistant to voltage, thereby making the on-state power loss larger.
Furthermore, in this case, the current flowing through primary winding 6a of transformer 6 increases, and time ratio D decreases; however, the loss becomes larger because of an increase in the effective value. Thus, a decrease in turns ratio N mainly increases the loss of the switching elements. In contrast, an increase in turns ratio N reduces the loss of the switching elements. However, the increased turns ratio of transformer 6 requires increasing the number of turns per layer or the number of layers when transformer 6 is made of a multilayer board provided with coils. As a result, it is inevitable that the transformer itself grows in size.
For example, when turns ratio N is set to 8 under an input voltage of 48V and an output voltage of 1V, time ratio D is 0.166. In order to set turns ratio N to 8, to make the primary winding area and the secondary winding area equal to each other, and to have one turn per layer, the multilayer board needs to have 16 layers. On the other hand, in order to form a multilayer board with 8 layers, the primary windings must have two turns per layer.
FIG. 7 shows an eight-layer board having primary windings of two turns per layer so as to make the transformer have windings at a ratio of 8:1:1. The board is provided with copper foil patterns 14 formed by etching or the like and spaces 15 corresponding to the cores to pass magnetic flux. Of throughholes A to I, throughholes having the same reference marks are on the same positions on each layer so as to connect between copper foil patterns 14 on the layers. Thus forming two turns per layer requires securing the distance between the patterns and providing interlayer connection of the multilayer board inside the coil areas. This develops a lot of spaces where no current can flow so as to reduce the usability, thereby leaving the problem of the growth of the transformer in size.
One such prior art technique is disclosed in Japanese Patent Unexamined Publication No. H06-215951.
This conventional structure has a problem in that when the windings of the transformer are made up of a multilayer board or a stack of planar conductors, a small turns ratio causes the loss of the switching elements, and increasing the turns ratio to reduce the loss increases the transformer in size.
The structure has another problem in that the use of the multilayer board or the stack of planer conductors increases the stray capacity between the coils, and the increased stray capacity allows a larger amount of noise to be transmitted by the switching voltage waveform generated by the on-off of the first and second switching elements, thereby deteriorating the stability.